Laser radiation is presently used in various devices by industry, commerce, medicine and armed services. In certain applications, the need exists for wavelength--tunable laser radiation having both high brightness and high energy capability. In the past optical parametric amplifiers (OPAS) and optical parametric oscillators (OPOs) have been used as devices for generating wavelength--tunable laser radiation with limited energy output. Prior art OPAs usually comprised an appropriately cut nonlinear birefringent crystal which amplified a weak "signal" beam by channeling energy from a strong "pump" beam of shorter wavelength. Two or more crystals were sometimes used in series to increase the OPA gain. In prior art devices the signal beam and pump beam usually arrive at the OPA colinearly. The difference of the photonenergies between the pump and signal beams is emitted in the form of an "idler" beam.
The parametric process is most effective when the "phase matching condition" is satisfied among the pump, signal and idler waves. The phase matching is satisfied by choosing appropriate angles between the crystal axes and the beam directions.
The optical parametric oscillator (OPO) is essentially the same as an OPA with a resonator cavity. In the case of an OPO, only a pump beam needs to be provided. The signal and idler beams are both generated within the resonator cavity. The wavelength of the generated signal idler pair are determined by the phase matching condition. The output wave-lengths may be tuned simply by changing the angle of the pump incidence, which is usually done by turning the crystal in the cavity.
One of the problems with the aforementioned is that all crystals, including those used for parametric conversion, have some laser intensity limit beyond which they damage. Since the size of the crystals that can be grown is limited, there is a limit on how much energy may be converted by a single aperature optical parametric array.
In the past when there was a need to frequency convert large amounts of pump laser energies one might have thought to use an array of many parallel OPAS. By using such a parallel arrangement one would scale up the energy conversion, but at the cost of ruining the beam phase fronts. The phase front degradation in a parallel OPA arrangement is caused by the fact that each individual OPA has slightly different lengths. The different lengths, when measured to a fraction of the laser wavelength, causes different parts of the original phase front to experience differing optical path lengths which leads to distortions of the phase front. The aforementioned phenomena of phase front degradation caused by differing optical path lengths is called "piston error". Phase front distortions results in an increase in beam divergence and lower brightness. This condition is generally unacceptable because for most laser applications the beam brightness is of central importance. A similar problem occurs in scaling up laser amplifiers.